1. Field of the Invention
The present invention relates to an improved machine and method for assembling high voltage fuses without an internal core, which fuses comprise at least one fusible element helically wound inside a tubular casing made of an electrically insulating material and filled with a pulverulent arc-quenching filler.
2. Discussion of Related Art
A known method conventionally used for assembling high voltage fuses consists of winding one or several fusible elements, usually made of silver, around a rigid core made of an insulating material such as ceramic, steatite or mylar. The fusible elements are then inserted with their supporting core inside a tubular casing which is subsequently filled with quartz and/or any other pulverulent filler capable of absorbing the energy produced when a fault current is interrupted or switched off. After the tube is filled, each of its ends is sealed with a cap.
This type of high voltage fuse, which is provided with a supporting core, has proven to be quite efficient up to now. Unfortunately, it has also proven to have at least two major drawbacks.
First of all, when the fuse is subjected to an overload current for a long period of time, its internal temperature increases in a non-uniform manner. The voltage distribution inside the fuse is then affected by the presence of the core whose thermic coefficient and dielectric constant are different from the thermic coefficient and the dielectric constant of the pulverulent filler. This first drawback results in practice in a substantially lower stability of the fuse when a current is switched off.
Secondly, the presence of the core inside the fuse substantially increases the cost of manufacture of the fuse.
To obviate these two drawbacks, it has been proposed to manufacture and use high voltage fuses without a core in order to improve the current breaking ability of the fuse, to insure a better electrical restoration after break and to reduce the cost of manufacture. Machines and methods for assembling such high voltage fuses without a core are described by way of example, in U.S. Pat. Nos. 3,831,251 of 1974; 3,839,786 of 1974; 3,848,214 of 1974; 3,959,875 of 1976; 4,003,129 of 1977; and 4,008,451 of 1977. A similar method of manufacture is also described by way of example in German Pat. No. 601,752 of 1934.
If the machines and methods described in these patents all have some advantages, they also have the common drawback of being very specific. That is, they are each restricted to the manufacture of a very specific kind of fuse, whereas it is known that the manufacture of fuses having a small nominal value is different from the manufacture of fuses having a high nominal value; the former requiring only a small fusible element while the latter, which are generally also designed for breaking small overload currents, require a plurality of fusible elements whose relative spacing inside the casing easily reaches a critical minimal value that must be strictly controlled.
It is also known that the manufacture of high voltage fuses without a core involves technical problems that are completely different depending on the shape, size and mechanical characteristics of the fusible elements. Thus, by way of example, the technical problems encountered when using circular wires or rectangular wires as fusible elements are completely different. Similarly, the problems encountered when manufacturing fuses are completely different if the mechanical characteristics of the metal used for the fusible elements or the number of elements inserted inside each tubular casing are different.
As previously indicated, most of the machines or methods proposed up to now for assembling high voltage fuses without a core are, unfortunately, applicable only to a small number of fusible elements. Also, the known machines and methods require a lot of skill from their operators and cannot be automated easily. Furthermore, they give very little freedom to fuse designers for selecting the structural elements of the fuses and they also often generate a plurality of constraints which must be met when assembling the fuses, especially in view of the fact that it is well known that the fusible elements must be helically wound inside the tubular casing to obtain fuses having a reasonable and useful size.